Re: possible bug: libnsgif does not handle successive RESTORE frames correctly

Here's a possible patch:

https://github.com/libvips/libvips/commit/bf522eeffc5aa84a275b1d53e24d0daace59fa3e


On Sun, 7 Mar 2021 at 14:31, <jcupitt@gmail.com> wrote:
>
> Hello again everyone,
>
> I think I have a GIF which libnsgif does not handle correctly:
>
> https://github.com/libvips/libvips/blob/master/test/test-suite/images/dispose-previous.gif
>
> This GIF has three frames which draw three concentric red circles. The
> GIF does this:
>
> - clear frame
> frame 0:
> - disposal_method COMBINE
> - draw outermost red circle
> frame 1:
> - disposal_method RESTORE
> - draw middle red circle
> frame 2:
> - disposal_method RESTORE
> - draw inner red circle
>
> So the viewer should see this:
>
> https://github.com/libvips/libvips/blob/master/test/test-suite/images/dispose-previous.png
>
> frame 0:
> - outer circle
> frame 1:
> - outer + middle
> frame 2:
> - outer + inner
>
> However, if you view with libnsgif, you'll see frame 2 has all three
> circles, ie. the middle frame is not removed.
>
> Looking at the code, the problem seems to be that
> gif_internal_decode_frame() calls gif__record_previous_frame() before
> it calls gif__recover_previous_frame(). This means that if you have
> two frames with RESTORE dispose in a row, the second restore will be
> missed, since it will save the frame (overwriting the restore) before
> it tries to restore it.
>
> The simplest fix is probably to simply move the record handler to just
> after the recover. I think this makes intuitive sense -- you want to
> finish disposing of the previous frame before you record the current
> state.
>
> John
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